Polyolefin resins, particularly propylene resins, are excellent in electrical properties, mechanical properties, chemical properties, formability, cleanness and recyclability, and thus are used in large quantities mainly for automobiles, in electrical fields, and for packages and commodities. Generally, however, polyolefin resins have a disadvantage that coating and adhesion are difficult because it does not contain a polar group in the molecular chain. Thus, for coating and adhesion of propylene resin, a method using a resin containing a chlorinated polypropylene resin as a main component has been proposed. However, because chlorinated resins release harmful substances such as acid gas when burnt, it is recently strongly desired to make the shift to non-chlorine materials with increasing awareness of the environment. Thus, development of modified polyolefin resin modified by acid or the like is under way.
To use modified polyolefin resin for coating and adhesion, the resin must be liquefied, and for example, methods have been employed, such as a) using resin after melting, b) using resin dissolved or dispersed in an organic solvent and c) using resin dispersed in an aqueous medium. a), however, involves problems of limited application and difficulty in forming thin layer with respect to the melt viscosity of the resin. And when b) is compared with c), c) is preferred because use of organic solvent tends to be restricted by policies of environmental protection, resource saving, regulation of hazardous materials by law and improvement of work environment.
Under the above circumstances, preparation of an aqueous dispersion of a modified polyolefin resin has been actively studied. For example, JP-A-6-73250, JP-A-6-80738, JP-A-6-80844, JP-A-6-80845, JP-A-6-256592, JP-A-8-3376, JP-A-8-67726, JP-A-8-92427 and JP-B-2895574 disclose that various surfactants are used as essential components for dispersing a modified polyolefin resin (mainly polypropylene) in an aqueous medium in a stable manner. In addition, JP-B-2610559 discloses use of liquid polyol as an essential component.
However, surfactants as described above are non-volatile and remain in the coating film of the modified polyolefin resin even after drying, and when used in large quantities, they greatly reduce the water resistance of the coating film. Even in small amounts, interfacial conditions are dramatically changed and properties such as adhesion to a substrate are adversely affected. Moreover, the surfactant may bleed out from the coating film, and this not only is environmentally or hygienically detrimental but also may result in change in properties of the coating film overtime. Furthermore, because liquid polyol is highly hydrophilic, it is difficult to complete preparation of resin aqueous dispersion only by polyol. Actually, a surfactant is used together in JP-B-2610559. In that case, the above-described problems arise. Even if preparation of resin aqueous dispersion is successful only by liquid polyol by increasing the carboxyl group content of the resin, there is another problem. Specifically, because polyol has a high boiling point, it remains in a coating film in low temperature, short time drying and adversely affects properties of the coating film such as water resistance and adhesion to a substrate. Further, polyol may bleed out from the coating film, and this not only is environmentally or hygienically detrimental, but also may result in change in the properties of the coating film overtime. Moreover, because high temperature and long time drying is necessary for evaporating polyol, this technique is industrially disadvantageous and involves a problem of deformation of the substrate depending on conditions.
Under such circumstances, the present applicant has proposed a modified aqueous polyolefin resin dispersion which does not contain a non-volatile water-compatibilizing agent such as surfactant (JP-A-2003-119328). However, since modified polyethylene resin is used in this dispersion, adhesion to polypropylene resin needs to be improved although adhesion to nylon and polyethylene is excellent.
Now, water-based coating material is described. Non-chlorine polyolefin resins described above have also been applied to a water-based coating material as a binder. In particular, non-chlorine, water-based types are greatly demanded in food packaging films. Usually, food packaging films are prepared by applying a coating material on a substrate film and laminating a polyolefin resin film or the like.
On the other hand, lamination techniques include a method comprising applying a coating material to a substrate film, applying an anchor coating agent to the coated surface and laminating a molten resin on the anchor coat layer as an adhesive layer (extrusion lamination method), a method comprising laminating various films on the anchor coat layer as an adhesive layer (dry lamination method), and a method in which molten resin is directly laminated on the coated surface of the substrate film without using an anchor coating agent (hereinafter referred to as “direct lamination method”). Of these, the direct lamination method is advantageous from the aspects of economical efficiency, resource saving and environment.
In such situation, as coating materials using non-chlorine polyolefin resin, those described in JP-A-8-120205, JP-A-2002-226758 and JP-A-2003-313483 are known.
However, the coating materials described in these publications have a problem that they have poor adhesion to a propylene substrate when dried at a relatively low temperature of about room temperature. Further, those coating materials are still not suitable for direct lamination and need to be improved.